Turns Out Andromeda Actually Weighs The Same As Our Own Milky Way

Andromeda is the nearest major galaxy to the Milky Way and for a long time, it has been considered the bigger sibling of our own galaxy. New research suggests that view might be wrong.

According to Australian researchers, the Milky Way and Andromeda appear to have roughly the same mass, which is estimated to be around 800 billion Suns. The new estimation was possible thanks to high-precision measurements of the escape velocity of Andromeda, which allowed them to work out just how heavy our neighbor is. The study is published in the Monthly Notices of the Royal Astronomical Society.

“When a rocket is launched into space, it is thrown out with a speed of 11km/s to overcome the Earth’s gravitational pull,” lead author Dr Prajwal Kafle, from the International Centre for Radio Astronomy Research at the University of Western Australia, said in a statement. “Our home galaxy, the Milky Way, is over a trillion times heavier than our tiny planet Earth so to escape its gravitational pull we have to launch with a speed of 550km/s. We used this technique to tie down the mass of Andromeda.”

Dr Kafle did use the same approach in 2014 to constrain the mass of the Milky Wa, and, in that case, the mass was revised down from the previous estimates. In both cases, the researchers were able to understand a lot more about galaxy than the mass alone.

“By examining the orbits of high-speed stars, we discovered that this galaxy has far less dark matter than previously thought, and only a third of that uncovered in previous observations,” he explained.

The Milky Way and Andromeda are the largest members of the Local Group, the corner of the universe where we inhabit. The future of the two galaxies is also strongly linked. They will merge into a single object over the next 5 billion years. This new estimate changes how that collision will pan out.

“It completely transforms our understanding of the local group. We had thought there was one biggest galaxy and our own Milky Way was slightly smaller but that scenario has now completely changed,” Dr Kafle explained. “It’s really exciting that we’ve been able to come up with a new method and suddenly 50 years of collective understanding of the local group has been turned on its head.”

There is significant debate to the correct mass of both galaxies, but the new estimate has a small uncertainty compared to previous measurements. This makes a very strong case for a lighter Andromeda than previously thought.